Discounts on a customs value are not a default option within ShipHero. You will need to utilize automation rules to apply discounts to customs value. If no transaction of monetary value occurs, shipments are still subject to duties and taxes. Shipper/exporter is responsible for including customs value only statements on the customs or commercial invoice.
An automation rule will need to be created so that ShipHero knows how you want to apply a discount to the customs value: